feat(13-04): tag extraction across mp3/flac/m4a/ogg with media-root confinement

Completes Task 3 on top of the recovered wip checkpoint (be8f24b4):

- fix the programmatically-generated MP3 fixture's frame length: lofty's
  Header::read computes samples*bitrate*125/sample_rate with truncating
  integer division BEFORE adding the padding byte, so the FF FB 52 C4
  frame is 209 bytes, not 210 — the off-by-one made cmp_header miss the
  second frame sync and reject the whole file as containing an invalid
  frame (mp3_id3v24_yields_full_record now passes; fixture-only fix,
  production code untouched)
- all 7 music::tags tests green; no binary audio fixtures committed
  (fixtures are built byte-by-byte into tempdirs at test run time)
- extract_tags canonicalizes and confines to caller-supplied media_roots
  before opening any file (T-13-20); non-audio is a distinct NotAudio
  error vs the Ok/has_tags=false untagged fallback (T-13-21)
- entity types in music/mod.rs implement 13-MUSIC-MODEL.md exactly:
  hybrid-identity TrackId, derived albums/artists, MUSIC_SCHEMA_VERSION=1

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/music/tags.rs

@@ -165,10 +165,16 @@ mod tests {
     /// 64kbps, 44100Hz, mono, padded — the exact byte pattern lofty's own
     /// test suite uses as a known-good frame header
     /// (`mpeg/header.rs::tests::rev_search_for_frame_header`). Its computed
-    /// frame length (samples * bitrate * 125 / sample_rate + padding) is
-    /// 210 bytes.
+    /// frame length matches lofty's `Header::read` exactly:
+    /// `samples * bitrate * 125 / sample_rate` truncates *before* the
+    /// padding byte is added — `1152 * 64 * 125 / 44100 = 208`, `+ 1`
+    /// padding = **209** bytes. The frames written below must be exactly
+    /// this long, because lofty confirms a frame sync by comparing the
+    /// candidate header against the bytes found exactly `len` later
+    /// (`find_next_frame`/`cmp_header`); an off-by-one frame length makes
+    /// the whole file "contain an invalid frame".
     const MP3_FRAME_HEADER: [u8; 4] = [0xFF, 0xFB, 0x52, 0xC4];
-    const MP3_FRAME_LEN: usize = 210;
+    const MP3_FRAME_LEN: usize = 209;
 
     fn mp3_frame() -> Vec<u8> {
         let mut frame = MP3_FRAME_HEADER.to_vec();